The present invention relates to a method and a device for measurement of edges.
The invention is applicable to different types of edge, for example edges on a machined metal work piece which edges constitute a part of the circumference of the work piece or surround boreholes or other apertures in the work piece. Particularly, an aspect of the invention can be used for inspection of the projected length or area of a broken edge, i.e. for determining the so called break-edge value.
An aspect of the invention can also be applied for similar measurement of cutting edges of tools having for example hard metal inserts for machining work pieces, which cutting edges are subjected to wear such as flank wear, notch wear, etc. during machining, for example, during a turning or milling operation.
In the manufacturing industry where metal work pieces are subjected to different machining operations, such as reaming, drilling and milling, burrs are removed and very sharp edges are broken, for example, rounded, in a subsequent burring operation so as to obtain a desired shape of the edges. The burring operation may in turn comprise different steps, such as filing, buffing and tumbling. In the absence of such a subsequent treatment of the work piece in question, the sharp edges could cause cracks and damage of the work piece when being used as a structural member. Therefore, there are often tolerance requirements on the product which are based on stipulated maximum and minimum values for the edge break.
Furthermore, the performance of a cutting edge of an insert is very much dependent on the degree of wear. The type and degree of wear influence the characteristics, such as dimension, surface finish etc., of the final work piece machined by means of the insert. Hence it is desirable to continuously measure the wear of a cutting edge so as to ensure the cutting operation will give the desired result. Knowledge about the wear of a cutting edge can be used to calculate compensation displacement of a tool or to make a decision that an insert has to be indexed or replaced. There is also a demand for determining the size or shape of such cutting edges on inserts when the inserts are manufactured, before the use of the inserts in machining operations, so as to ensure the inserts have the requisite quality.
In accordance with prior art some kind of profile projector or microscope is often used for investigation of edges on work pieces in order to evaluate the result of a burring operation. Since, for practical reasons, it is difficult to apply these techniques on large work pieces which are not possible to be moved to a measurement station, a soft piece of aluminum can be used to create a reproduction of the edge and the adjacent area. By beating a soft aluminum piece against the edge to be measured, a reproduction sample is obtained. The aluminum piece is then removed and used for analysis purposes. The aluminum piece is easier to handle and can be put under a profile projector or a microscope for studying the profile and radii of the edge. However, in addition to be a very time-consuming method, due to the measurement on a reproduction sample which has an inverted shape in comparison to the real edge, the accuracy of the measurement of the length and shape of a broken edge when using this method is very often unsatisfying.
This in turn means that very few measurements of broken edges are performed. Besides the use of visual inspection to find any visually detectable defects or indications, usually, aluminum samples are only used as spot tests for checking the radius and/or the profile of the edge. In the manufacture of a certain component, very often measurements are performed only once or a few times when machining is started to ensure the size and the angle of the edge break are correct, and then the process is presumed to be stable during the remaining similar machining moments.
It is desirable to provide a method and a device of the type mentioned by way of introduction, which method and device enable automatically measurement of edges of work pieces or tools, particularly in situ measurements.
By a method/device according to an aspect of the present invention, the work piece can be measured without moving the work piece to a measurement station or using a reproduction sample of aluminum. Thanks to the provision of diffused radiation from several positions in the vicinity of the edge, it is ensured that a sufficient number of positions on the edge are subjected to radiation having such incident angles so that the radiation is reflected from the edge directly to the picture reproduction unit. Due to the geometrical shape of a component having an edge, it is possible to arrange the illumination means and the picture reproduction unit so as to illuminate the component in such a way that the edge surface is accentuated, whereas the adjoining surfaces which form the edge are suppressed in the produced picture of the edge. In other words; the radiation which reaches the surfaces surrounding the edge is mostly reflected in directions not receivable by the picture reproduction unit. The brighter the edge surface the more reflection where the incident angle is equal to the reflection angle is obtained. Thus, a sharp picture of the edge can be obtained, and this picture may be real-time digitalized for automatically evaluation. A picture of the edge can preferably be produced for measuring the projected length of the edge.
Although bright surfaces surrounding the edge are advantageous because such surfaces further accentuate the edge in the picture, an aspect of the invention can also be applied for evaluation of an edge which is surrounded by surfaces which are matt in comparison to the edge.
According to a preferred embodiment of an aspect of the invention, a diffusor plate is used to create a great number of individual radiation sources on the diffusor plate for illuminating an edge, each of which radiation sources radiates substantially in all directions in space which directions are defined by a semi-sphere for instance. Hereby, independent of the edge break angle, the edge may be illuminated by means of several radiation sources each emitting radiation which has various incident angles when reaching the edge. This ensures that there are always radiation beams which are reflected by the edge in directions directly towards the picture reproduction unit. The picture created by the picture reproduction unit is preferably based on direct reflection from the edge surface.
According to a further preferred embodiment of an aspect of the invention, a telecentric lens is used to enable 3-dimensional measurement with high accuracy. The use of a telecentric lens means that a substantially constant magnification in depth is obtained over a certain interval, i.e. the telecentric range. This in turn simplifies calibration of picture geometry and reproduction conditions, since the calibration has to be performed only once, as long as the dimension changes of the edge due to burring (or wear) not exceed the telecentric range,
Other advantageous features and functions of different embodiments of aspects of the invention appear from the following detailed description.